The culture of animal cells or tissues has been widely used not only in the field of medical transplantation but also in various fields of research and development including, as typical examples, gene analysis, bioreactor for producing useful products from cells, and evaluation of biological activity of agents or drugs.
At the scene of the current medical transplantation, for example, an urgent problem is a serious shortage of donors. Whichever donors are humans or animals, securing of “living donors” would be an extremely difficult problem even in the future.
On the other hand, as an extremely dominant measure for solving the above problem regarding the donor shortage, in vitro tissue engineering (Seitaigai Soshiki Kogaku) for producing organs, tissues or apparatuses for transplantation in vitro is in the spotlight. The basic strategy of this tissue engineering is that a cell (e.g., a stem cell) is incorporated into an artificial extracellular carrier (that is also referred to as “extracellular matrix” in the field of tissue engineering) together with a physiological active substance such as a growth factor as desired, so as to regenerate a specific organ, tissue or apparatus.
It has been conventionally considered that a carrier plays only a physical and structural role in the culture of a cell or tissue, such that it constitutes the framework of a tissue or organ, determines the form of the tissue or organ, and determines the hardness, strength and flexibility of the tissue or organ. However, with the progression of developmental biology and cell biology, it has been clarified that such an extracellular carrier has various regulatory actions on activities of cells such as differentiation, growth, transferring, adhesion, signal transmittance, gene expression, hormone action, or ion channel.
Under these circumstances, in view of the expectation of functions of extracellular carriers to differentiate or propagate cells, in vitro tissue engineering has been vigorously attempted by using various types of natural or synthetic extracellular carriers such as collagen sponge which has been obtained by freeze-drying bovine collagen Type I or biodegradable polymers such as polylactic acid or polyglycolic acid (e.g., “Tissue Engineering” edited by Minoru Ueda, 1999, published by the University of Nagoya Press may be referred to).
However, since the above described existing carriers used for cell growth are solid, they have problems in that it is difficult to inoculate (or seed) or mix cells or tissues into the carriers, or in that the growth or differentiation of cells are insufficient. In addition, there are other problems in that when it is difficult to dissolve the existing carrier for cell growth or when the existing carrier is dissolved to recover regenerated tissues therefrom, the tissues regenerated in the carrier are damaged because the carrier is heated to a high temperature to dissolve itself or is subjected to enzyme treatment so as to recover the regenerated tissues.
Moreover, the most serious problem regarding the conventional carrier is that since the growth of fibroblasts occurs more vigorously than the growth or differentiation of cells that are necessary for regeneration of a tissue or organ of interest in the above carrier, the regeneration of the tissue or organ of interest becomes difficult.